The exact remake of Super Mario Bros (World 1 Level 1).
Not an exact remake as claimed, but a pretty fair reproduction. Most issues were with collisions/hitboxes (def. not pixel-perfect), then wonky fireball behaviors (trajectories, frozen in-place), controls (no jump while running)....etc. Arguably minor things, but definitely noticeable and add up.
The tutorial series OP used was almost certainly the 20-part series by Nicolas Dingle on YouTube. I'd recommend people check it out; it's very well done. I'll just add that Dingle's final version isn't exact, either. ;)